A few weeks ago, I stumbled upon this quiz. I already noticed in the past that "its" and "it's" are commonly confused, but I never understood why: while I often make (pretty stupid) mistakes in English, I have difficulties understanding why this one is so common. Its^[1] quite easy to know when you need "its" and when you need "it's", at least that's my feeling. I can understand that this happens to people who are not fluent in English, but I keep wondering why so many native English people are confusing "its" and "it's"...

This is probably quite similar to what we can observe in France, though: many French people can't write correct French and I know non-native French speakers who speak better than most of us :-)